How Drugs Affect you

How drugs act on your body and their possible effects

A drug may have several types of effect on your body as well as the intended action. These include side effects, tolerance, and dependence. Interaction may also occur, in which drugs that are taken together enhance or reduce each other’s actions. Some drugs may also interact with certain foods and with complementary remedies. Many drugs can have a powerful psychological benefit called the placebo effect. A number of drugs have unwanted effects, which can be unpleasant or harmful, and your doctor will plan drug treatment to avoid or minimize these effects.

Side effects of drugs

Almost all systemic drugs (drugs that affect the whole body) can cause side effects – undesired reactions resulting from a normal dose. Many side effects occur because drugs act on cells throughout the body, not just in the area to be treated. For example, beta-blocker drugs may be used in the treatment of hypertension. However, they may disrupt sleeping patterns as a side effect of their intended action.

Some side effects, such as the dry mouth caused by some antihistamines, are predictable because they result from the known chemical effects of a drug. However, drugs may also produce unpredictable reactions such as drug allergy. Any type of drug, including penicillins (see Antibiotics), can cause allergic reactions that can range in severity from a mild rash to severe breathing problems. Other unexpected reactions occur in people whose genetic make-up influences their body’s ability to process particular drugs. For example, some people of Mediterranean, African, or Southeast Asian origin inherit a condition called G6PD deficiency, which affects the chemistry of red blood cells. If these people take certain types of drug, such as sulphonamides, they may develop haemolytic anaemia, a condition in which red blood cells are destroyed prematurely. As a result, the blood does not carry enough oxygen to body tissues.

Most side effects are not serious, and they often disappear gradually as your body becomes used to a drug. However, for some drugs used to treat serious disorders, the side effects are severe and potentially fatal. For instance, certain cytotoxic drugs used to treat cancer (see Anticancer Drugs) are toxic to the heart and can cause it to fail. A medical decision to use a drug depends on whether the overall benefit outweighs the risk of harmful effects.

Drug tolerance and dependence

If you take certain drugs for a long time, your body adapts to them in a process known as tolerance. With some drugs, tolerance may be useful, allowing the body to overcome side effects while still responding to the beneficial effects of the drug. However, tolerance may make some drugs less effective so that a higher dose is needed to obtain the same result. The higher dose may increase side effects.

Dependence is a need for a drug. The need can be psychological or physical. If you become dependent on certain drugs, your body may develop tolerance to them. If you then stop taking them, you may suffer unpleasant effects known as withdrawal symptoms.

People at special risk

The effects of a drug may differ from one person to another. This variation occurs because people’s bodies absorb and excrete drugs at different rates. In addition, the same dose of a drug may reach different concentrations in the blood depending on factors such as body size and kidney function.


Most drugs that are taken during pregnancy pass across the placenta to the fetus. Many can harm the fetus, especially if taken during the first 12 weeks of pregnancy when the fetus’s organs are developing. If you know or believe that you are pregnant, check with your doctor or pharmacist before taking any drug, including over-the-counter drugs. If you take medication for a long-term condition, consult your doctor if you are planning a pregnancy.

Breast-fed babies

If a woman takes drugs while breast-feeding, they may pass into her breast milk. Some drugs cause unwanted effects in the baby. For example, antianxiety drugs may make the baby drowsy. If you are breast-feeding, you should check with your doctor or pharmacist before taking any drug, including over-the-counter drugs.

Babies and children

Drugs must be used with care in babies and children. Children’s doses are usually smaller than doses for adults and are often calculated on the basis of a child’s weight or age. It is very important to give children the correct dose. Never give a child a drug prescribed for an adult.

People with liver or kidney disease

Most drugs are broken down by enzymes in the liver. The drugs are then eliminated by the liver or the kidneys and excreted in the urine. If your liver or kidneys are not functioning well, toxic substances may build up in your blood, increasing the risk of side effects. You may need lower doses than normal. Alcohol also affects liver function, so the effect of some drugs may be altered if you drink heavily or regularly.

Older people

Older people are at in-creased risk of side effects. This risk may be due to the decline in the function of organs such as the liver and kidneys as the body ages, which causes toxins to accumulate faster in the body.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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